I. Fillers
Polymers commonly used in adhesives, coatings, sealants, and the like generally have high CTEs when compared to glasses, ceramics, metals, and metal oxides. This mismatch in expansion creates internal and interfacial stresses which often cause either delamination or poor product performance. The problem is the greatest at temperatures lower than the glass transition temperature (T.sub.g) of the polymer where the polymer has its greatest modulus and thus cannot effectively release stresses through creep. As a result, at its in-use temperature, the polymer is typically in tension, while the lower CTE substrate is in compression.
A common approach to this problem is to add inorganic fillers having a lower CTE to the polymer. In some cases, special fillers with negative CTEs on the order of -5.times.10.sup.-7 /.degree. C. have been used, such as lithium aluminum silicates. Notwithstanding these efforts, polymeric compositions having CTEs close to zero or below zero (i.e., negative CTEs) have been difficult to achieve, even when large amounts of filler have been employed.
Examples of fillers which have been added to polymers of various types can be found in: C. F. Hofmann, U.S. Pat. No. 3,547,871, W. A. Ernst, et al., U.S. Pat. No. 3,568,012, M. Tsukui, et al., U.S. Pat. No. 3,658,750, C. R. Sporck, U.S. Pat. No. 4,043,969, Chenoweth et al., U.S. Pat. No. 4,104,238, Shinohara et al., U.S. Pat. No. 4,358,552, Miller et al., U.S. Pat. No. 4,987,274, Cheilis et al., U.S. Pat. No. 5,126,192, and H. Bayer, et al., U.S. Pat. No. 5,158,990.
II. Crystallized Phosphate Glasses
Crystallized phosphate glasses which undergo a crystal (phase) inversion have been used to control the CTEs of lead and tin-phosphorous oxyfluoride sealing glasses. See Cornelius et al., U.S. Pat. No. 5,089,446 (tin-phosphorous oxyfluoride sealing glass), and Francis U.S. Pat. No. 5,089,445 (lead sealing glass).
In connection with these applications, the crystallized phosphate glass and the sealing glass, in particular, a tin-phosphorous oxyfluoride sealing glass, have been compounded with an organic medium to form a sealing composition, e.g., a sealing tape. See Francis et al., U.S. Pat. No. 5, 179,046.
The organic medium included in these sealing compositions serves as a binder (carrier) for the sealing glass and the crystallized phosphate glass. The binder is specifically designed to vaporize as the sealing composition is used, i.e., the binder is chosen to have a high vapor pressure within the operating range of the sealing composition. In particular, the Francis et al. patent specifies that the binder should have a boiling point below 350.degree. C.
In contrast, the polymers of the present invention constitute an essential pad of the finished, in-use composition and thus do not substantially vaporize within the composition's operating range. That is, the organic medium of the Francis et al. patent was a fugitive medium, while the polymers of the present invention are non-fugitive. Moreover, in the Francis et al. patent, the crystallized phosphate glass does not serve to control the CTE of the binder, but rather is designed to control the CTE of the sealing glass after the binder has been vaporized. As discussed below, in accordance with the present invention, crystallized phosphate glasses are used to control (reduce) the CTEs of polymer compositions, an application very different from that of the foregoing Francis, Cornelius et al., and Francis et al. patents.
III. Optical Waveguide Couplers
The use of adhesives in the assembly of optical waveguide couplers is disclosed in, for example, Miller et al., U.S. Pat. No. 5,009,692. Adhesives have also been used in the formation of integrated optical components in which an optical waveguide formed in a substrate is coupled to one end of an optical waveguide fiber. See Vial et al., U.S. Pat. No. 5,185,835.
The use of filled adhesives having low CTEs in the construction of optical waveguide couplers is discussed in EPO Patent Publication No. 553,492. This publication discloses a variety of fillers for use in such adhesives, including fillers composed of lithium aluminum silicates which have a negative CTE. The reference, however, does not disclose or suggest the use of crystallized phosphate glasses for this purpose.